Neptune 4: first layer too far or squashed after leveling, or the print head does not stop going down
Guide based on the manufacturer's official documentation; usi3d has not yet verified it on a machine.
Technology: FDM · Brand: Elegoo · Symptoms: Uneven first layer (thin and thick areas), Unlevel or warped bed, Homing or a sensor fails (endstop, levelling) · Updated on 9 October 2026
Symptom
On the Neptune 4 or 4 Pro, after automatic leveling the nozzle is too close to or too far from the bed and the first layer comes out wrong; you have to level every time; in auxiliary leveling the nozzle cannot reach the bed even with the nut fully loosened; or, when homing, the print head keeps going down in Z and does not stop.
Likely causes, from most to least likely
- The metal PEI plate is not on the bed: the proximity sensor only detects metal and does not stop the print head.
- The Z offset is set wrongly for your machine.
- Mechanical adjustment: POM wheels overtightened, Z brass nut screws fully tightened, play in X or Y, loose bed screws or an X gantry that is not parallel to the bed.
- The proximity sensor is tilted or loose, its connector is loose or it has bent pins.
- A non-original nozzle has been fitted: the Neptune 4 and 4 Pro nozzle is a custom size.
- Thermal drift affects the sensor reading when leveling with a cold bed.
Checks to tell which one it is
- Check that the PEI plate is fitted before leveling or homing.
- Move the print head and the bed by hand: they must have no play.
- Measure with a ruler the distance from each end of the X gantry to the bed and compare them.
- With the printer off, check whether the proximity sensor is tilted or loose and whether its connector is firmly seated.
Before you start
Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.
Step-by-step fix
Fit the PEI plate and the original nozzle
Always fit the metal PEI plate before leveling or homing: the proximity sensor only detects metal. Use the original nozzle, because the Neptune 4 and 4 Pro nozzle is a custom size.
Check: When homing, the print head stops as it approaches the bed.
If that does not fix it: Check the proximity sensor (next step).
Check the proximity sensor
Switch the printer off and unplug the power cable. Check that the connector pins are not bent, that the sensor connector is firmly seated (also on the mainboard, under the bottom cover) and that the sensor is not tilted and its mounting screw is not loose.
Check: The sensor sits straight and firm, and homing stops at the right height.
If that does not fix it: If the print head still does not stop, contact Elegoo technical support; according to Elegoo, these checks do not void the warranty.
Remove play and straighten the gantry
With the printer off and unplugged, move the print head and the bed by hand. If they have play, turn the eccentric nuts with an open-end wrench a third of a turn at a time without overtightening; the POM wheels must touch the extrusion and roll smoothly. Leave the Z brass nut screws slightly loose. Remove the PEI and tighten the screws between the bed and its bracket. If the X gantry is not parallel to the bed, correct it.
Check: The print head and bed move without play or binding and the gantry is at the same height on both sides.
If that does not fix it: Repeat auxiliary leveling (next step).
Auxiliary leveling and Z offset
Run auxiliary leveling with a sheet of A4 paper between nozzle and bed at the four points, turning the nut until you feel slight friction. If the nozzle cannot reach the bed even with the nut fully loosened, return the nut to its initial position and lower the Z offset on that screen (it is usually negative; Elegoo suggests trying about -1 mm) before repeating. Then let it run automatic leveling and save the correct Z offset.
Check: With the offset saved, the paper pulls out with friction but can no longer be pushed in, and the first layer sticks well.
If that does not fix it: Level with the bed hot (next step).
Level with the bed hot
Thermal drift alters the sensor reading. Set the bed to 60 °C, wait about 20 minutes for it to stabilise and repeat automatic leveling and the Z offset adjustment.
Check: The Z offset holds between prints and you no longer need to level every time.
If that does not fix it: Update the firmware (next step).
Update the firmware
Install Elegoo's latest firmware, which according to the manufacturer improves leveling, and repeat leveling and the Z offset adjustment.
Check: Leveling holds from one print to the next.
If that does not fix it: Contact Elegoo technical support with the serial number (on the white label under the bed, lower left corner), the firmware version (Settings → About Printer) and the result of each check.
Prevention
- Do not level or home without the PEI plate fitted.
- Level with the bed already hot to avoid sensor thermal drift.
- After shipping, check for play and check the wheels and bed screws: Elegoo notes they can come loose in transit.
- Before switching on, check that the voltage selector on the back is set to your country's mains voltage.
Still not fixed?
Tell usi3d's AI about your case: choose your printer, describe what you see and add photos if you like. It proposes checks step by step, with their sources.
Printers it applies to
Elegoo Neptune 4 and Neptune 4 Pro.
Official sources
This guide summarises the official documentation linked here:
This guide is general information: usi3d has summarised it from the official documentation linked here, and it may not match your model, its version or its firmware. If in doubt, follow the manual and the support of your printer's manufacturer. You decide what you do with your machine, and working on it may affect its warranty.